Started playing music in 3rd grade (age 7) when my folks signed me up in the school music program, which was probably the best parenting decision they ever made. I wanted to play cello but my mom thought it was too big to lug around in her hatchback Honda Civic so she convinced me to play violin, which I only stuck with for a year. Fourth grade, switched to clarinet (in the LBUSD they didn't start teaching brass/woodwinds til 4th grade), which I grew to love, and also joined glee club, which I did for two years. Regretted stopping singing at that point when I later tried to sing again in my 20s, but played clarinet through middle school where I peaked playing the city honor band in 8th grade. Stopped playing clarinet in school in high school since I didn't want to do marching band, it wasn't a jazz instrument, and symphony band conflicted with other electives I wanted to do, but I also started teaching myself guitar at this age and writing my own songs and started a garage band with some friends. Also took a year of school piano lessons during high school and I continue to play piano poorly.
Since college and hitting my 20s, I've played on and off with various non successful bands, done some backing work on the bass (at some point I bought a bass so I could record it myself at home and people like my bass playing), started DJing a bit at the suggestion of a DJ friend who liked my music taste, kept recording at home (but not frequently enough), and spent way too much time clubbing and going to concerts, which has the nice side effect of greatly expanding your musical network. Even if I haven't been hugely successful like some friends I have, I can't even imagine not having music as part of my daily life and it's helped me get through some tough times in other aspects of my life.
Most of the other songwriting suggestions on here are pretty much on the spot and things I go to as well. Sometimes I surprised how much I go back to some of the basics I learned back in elementary and middle school, not just scales and stuff like that but how I practice and organize music in my head. I'm not the biggest music theory nerd in the world but having some background in that greatly helps when you're talking to other musicians or trying to pick up something new as far as reducing the learning curve.